1. Field of the Invention
This invention relates to a method for producing reduced iron by heat-reducing an iron oxide such as iron ore with a carbonaceous reducing agent such as coke to obtain solid reduced iron, or further heating it, thereby separating a slag forming component as slag from the metallic iron to obtain granular metallic iron.
2. Description of the Background
As methods for producing reduced iron similar to this invention, it is known to pelletize raw material powders containing an iron oxide source such as iron ore and coke, and charge the raw material agglomerate into a heat reduction furnace as it is in an undried state to heat-reduce the green pellets, thereby successively performing the drying and heat reduction to produce reduced iron. Although this method has the advantage that the equipment or time required for drying the raw material agglomerates can be omitted, it requires a preheating zone serving also as drying zone before the heat reduction area, resulting in the inevitable enlargement of the whole furnace. It further requires the preparation of a shielding member such as curtain wall in order to obstruct the flow of a high-temperature gas from the heat reduction zone toward the preheating zone, and the structure of the furnace is consequently complicated, leading to the problem of an increase in equipment cost.
It is also known to enhance the heating efficiency by shaping the raw material layer charged on the hearth into ridges to extend the surface area of the raw material layer. In this method, however, the rate of heat transfer of burner heat or radiation heat into pellets is low even in raw material pellets having middle to large particle sizes of 10-20 mm, the pellets are barely stacked in several layers for all the formation of ridges, and a sufficient heat transfer effect cannot be necessarily obtained. It is further known to enhance the heating efficiency by plowing up the ridges in the middle of the heat reduction. However, the plowing up of the laminated part of the middle to large particle size pellets causes the breakage of the pellets, resulting in a reduction in yield of reduced iron.
Further, it is also proposed to supply the raw material powders onto the hearth while forming irregularities. However, the heat transfer property or reduction reactivity of this method is rather inferior, compared with the use of the agglomerate because the maximum accumulation thickness of the raw material is large, and the iron oxide source and the carbonaceous material of the raw material powders are only mixed together but not so closely in contact to each other.
In these methods, the raw material mixture is generally molded into a agglomerate having a diameter of about 10 mm or more, and it is supplied onto the hearth of a heat reduction furnace and heat-reduced. Since the raw material agglomerate having such a large diameter is apt to rupture by the influence of the moisture or volatile component contained therein when exposed to a high temperature of about 1300xc2x0 C. or higher for efficiently progressing the reduction reaction. In most cases, therefore, the raw material agglomerate is preliminarily heated and then charged into the heat reduction furnace.
Further, the large size raw material agglomerate is generally difficult to pelletize, resulting in not only an increase in the cost required for pelletizing equipment or drying equipment but also an increase in the production cost. A binder is used in order to stably retain the shape after drying. However, an excessively large mixing quantity of the binder tends to hinder the uniform dispersion of the iron oxide source and carbonaceous material in the agglomerate and also causes the fear of affecting the efficiency of the heat reduction reaction. It is also proposed to omit the drying and supply the agglomerate to the heat reduction furnace in a green pellet state. However, this method cannot be said to be practically applicable in industrial scale because the green pellet is not only low in strength but also apt to cause a clogging by the mutual adhesion of the pellets or the adhesion to the hopper of a feeder with poor handling property.
The reduced iron obtained by the method as described above has a low Fe purity because a large quantity of the slag component included as gangue component in the raw material iron ore is contained therein, and requires the process for removing the slag component in the following refining treatment process. Further, the reduced iron obtained by this method is lacking in handling property in the merchandising as iron source because it is spongy and easy to break. To improve such a disadvantage, the spongy reduced iron must be worked into a briquette-like compact, which requires an extra apparatus.
Therefore, it is proposed to melt the metallic iron produced successively to the heat reduction of the reduced iron and coagulate the molten metallic iron while separating from the by-produced slag component to obtain granular metallic iron. However, a sufficient examination has not be necessarily performed in this method for how efficiently granular metallic iron is produced, taking into account the size or the like of the raw material agglomerate.
This invention has an object to provide a method capable of stably and efficiently performing the agglomerating of a raw material, the drying and heat reduction, and further the melting and coagulating by properly setting, particularly, the size or number of layers of raw material small agglomerates in the production of solid reduced iron (granular metallic iron, reduced iron containing slag) from a raw material containing an iron oxide source and a carbonaceous reducing agent.
Namely, a method for producing reduced iron according to this invention comprises agglomerating a raw material mixture containing a carbonaceous reducing agent and an iron oxide containing material into small agglomerates, charging the small agglomerates into a reduction furnace, and heating the small agglomerates in the reduction furnace, thereby solid reducing the iron oxide in the small agglomerates to produce solid reduced iron.
In the above method for producing reduced iron, the small agglomerates are mainly composed of those having particle sizes of less than 6 mm, or particle sizes of 3 mm or more and less than 6 mm, and the small agglomerates are charged in 2-5 layers depth.
In the above method for producing reduced iron, the small agglomerates are mainly composed of those having particle sizes of less than 3 mm, and the small agglomerates are charged onto the hearth of the reduction furnace in 3 layers depth or more.
In the above method for producing reduced iron, the small agglomerates are mainly composed of those having particle sizes of 3-7 mm, and the small agglomerates are charged onto the hearth of the reduction furnace so as to mutually overlap in a thickness of 10-30 mm.
In the above method for producing reduced iron, the small agglomerates are leveled so as to have 3-5 layers depth.
In the above method for producing reduced iron, the small agglomerates are charged into the reduction furnace without drying, the small agglomerates are charged onto the hearth after drying at least the surface thereof, mountain parts and valley parts are formed on the surface of the small mass layer charged on the hearth of the reduction furnace, the small agglomerates are charged after a powdery carbonaceous material is laid over the hearth of the reduction furnace, or the small agglomerates are charged onto the hearth with the carbonaceous material being adhered to the surface thereof.
According to this, the small agglomerates are used as the raw material, whereby the agglomerating of the raw material, the drying and heat reduction and further the melting can be stably and efficiently performed. The preferable particle size is 3-7 mm or less than 6 mm, but the particle sizes of the small agglomerates are more preferably set to 3 mm or more and less than 6 mm. In this case, the small agglomerates are charged on the hearth in a thickness of 10-30 mm or in 2-5 layers depth, preferably, 3-5 layers depth, whereby the productivity as product reduced iron can be sufficiently enhanced. Further, the particle sizes may be less than 3 mm. In this case, the small agglomerates are desirably charged in 3 layers depth or more in order to improve the productivity. When small agglomerates having such small particle sizes are used, the production of reduced iron can be efficiently performed under stable operability without causing the rupture or crush of the small agglomerates even if the small agglomerates are charged into the heat reduction furnace in an undried state without drying or in a semi-dried state.
The small agglomerates are charged into the heat reduction furnace with the carbonaceous powder being adhered to the surface thereof, whereby the erosion of the hearth refractory material by the molten slag produced in reduction process derivatively from the gangue component in the raw material can be suppressed, and the reoxidation of reduced iron in the last stage of reduction can be also preferably prevented. Further, when a small size raw material agglomerate having a high crushing strength, compared with a large size agglomerate, is used, the agglomerate can be placed on the hearth so as to have, for example, 3-5 layers depth and heat-reduced without stopping, whereby the productivity can be more enhanced. At this time, mountain parts and valley parts are irregularly formed (ex. convex and concave) on the surface of the small mass layer charged on the hearth, whereby the heat from above can be more efficiently transferred to each small mass on account of the enlargement of the effective heat transfer surface area of the small mass layer, and the heat transfer to the lower layer-side small agglomerates can be also hastened to further enhance the productivity.
It is also recommended to adapt the method of charging the small agglomerates onto the hearth after drying at least the surface thereof since the supply failure by the mutual adhesion of the small agglomerates in a hopper part in the charging of the small agglomerates to the furnace or the crushing of the agglomerates by the stacking load after charge can be further prevented.
This invention further involves a method for producing reduced iron comprising agglomerating small agglomerates of particle sizes of 10 mm or less containing a carbonaceous reducing agent and an iron oxide containing material, charging the small agglomerates into a reduction furnace so as to have the number of layers determined from operation conditions, for example, the number of layers (H) satisfying the following relation, and heating the small agglomerates in the reduction furnace, thereby solid reducing the iron oxide in the small agglomerates to produce solid reduced iron:
xe2x80x83H=Zxc3x97[Xxc3x97(G/P)]/[Axc3x97LOAD÷T]
wherein H is the number of layers of the small agglomerates, X is the productivity (kg/min) of the heat reduction furnace, Z is a positive number ranging from 0.7 to 1.3, LOAD is the mass per unit area (kg/m2) of the small agglomerates in the charging over the hearth in one layer, G/P is the mass ratio of the charging quantity of the small agglomerates to the reduced iron to be discharged, and A is the furnace floor area (m2) for charging the small agglomerates, T represents the production time (min) in the productivity X.
In the above method for producing reduced iron, the particle sizes of the small agglomerates are 6-10 mm, the small agglomerates are charged onto the hearth of the reduction furnace in 1-3 layers depth, the particle sizes of the small agglomerates are homogenized within xc2x13 mm, and the surface temperature is raised to 1200xc2x0 C. or higher in the time of ⅓ of the total reduction time after the small agglomerates are charged into the reduction furnace.
According to this, when the upper limit of particle size of the raw material agglomerate is set to 10 mm, the number of layers H of the raw material agglomerate charged on the hearth is specified so as to satisfy the relation of the above expression. In the range satisfying such a relational expression, the productivity of reduced iron can be significantly enhanced, compared with in the past even if the particle size of the raw material agglomerate is within the range of 6-10 mm. The raw material agglomerate desirably has a narrow particle size distribution, and those having particle sizes preferably within the range of xc2x13 mm, more preferably within the range of xc2x12 mm are used, whereby the operation stability and the productivity as reduced iron can be further enhanced. Further, the surface temperature of the raw material agglomerate is preferably raised to 1200xc2x0 C. or higher in the time of ⅓ of the total reduction time after charged into the heating reduction furnace, whereby the reduction can be efficiently proceeded in a short time.
In the method for producing reduced iron according to this invention, the small agglomerates are preferably mainly composed of those having particle sizes of 3 mm or more and less than 6 mm, or 3-7 mm. In the heat reduction by use of such small agglomerates as the raw material, the small agglomerates are preferably charged onto the hearth of the reduction melting furnace so as to mutually overlap in a thickness of 10-30 mm (about 3-10 layers), whereby the productivity of granular metallic iron can be further enhanced. Further, mountain parts and valley parts are preferably formed on the surface of the small mass layer to irregularly charge the small agglomerates onto the hearth, whereby the heating efficiency can be enhanced by the extension of the heat transfer effective surface area, and the heating speed of the small agglomerates in the stacking lower layer part can be also enhanced to more efficiently perform the whole reduction and melting.
Further, when the method of charging the small agglomerates after laying a powdery carbonaceous material over the hearth, or charging the small agglomerates onto the hearth after adhering the carbonaceous powder to the surface thereof is adapted, the carbonaceous material is carburized to the metallic iron produced by reduction to lower its melting point, so that not only the melting of metallic iron can be more efficiently progressed, but also the adhesion to the hearth surface of the molten metallic iron produced by melting can be suppressed to promote the granulation of the molten metallic iron by coagulation. Further, the erosion of the hearth refractory material by the molten slag rich in FeO easily produced by the insufficient reduction in the stacking bottom layer part of the small agglomerates charged on the hearth can be also suppressed.